PPP & Serial

Point-to-Point Protocol — the Layer-2 encapsulation for serial / dial links. LCP brings the link up, optional PAP/CHAP authenticates, then an NCP (IPCP) configures Layer 3.

// connection phases

Link Dead

→

Establish
LCP

→

Authenticate
PAP / CHAP

→

Network
NCP / IPCP

→

Open

→

Terminate
LCP

Authentication failure → straight to Terminate. Termination returns the link to Link Dead.

1	Link Dead	Physical layer down; carrier detect triggers establishment
2	Link Establishment	LCP negotiates options (MRU, auth method, magic number)
3	Authentication	Optional — PAP or CHAP must succeed to continue
4	Network-Layer	NCP (e.g. IPCP) configures L3 and assigns addresses
5	Link Termination	LCP closes the link → returns to Link Dead

// PPP frame

Flag

1 B · 0x7E

Address

1 B · 0xFF

Control

1 B · 0x03

Protocol

2 B · —

Information

var B · —

FCS

2 / 4 B · —

Protocol identifies the payload (LCP, IPCP, IP…). Address/Control are fixed (0xFF / 0x03). FCS is the CRC.

// LCP header

Code

1 B

Identifier

1 B

Length

2 B

Data

var B

// LCP codes

1	Configure-Request
2	Configure-Ack
3	Configure-Nak
4	Configure-Reject
5	Terminate-Request
6	Terminate-Ack
7	Code-Reject
8	Protocol-Reject
9	Echo-Request
10	Echo-Reply
11	Discard-Request

// protocol field values

0xC021	LCP
0xC023	PAP
0xC223	CHAP
0x8021	IPCP
0x0021	IPv4
0x8057	IPv6CP
0x0057	IPv6

// authentication — PAP vs CHAP

	PAP	CHAP
Method	2-way handshake	3-way challenge
Secret	Sent in cleartext	Never sent — MD5 hash of challenge + secret
When	Once at link-up	Link-up + periodic re-challenge
PPP protocol	0xC023	0xC223

// connection example (CHAP)

R1R2

LCP — establish

Configure-Request

▶

Configure-Ack

◀

CHAP — authenticate

Challenge

◀

Response (MD5 hash)

▶

Success

◀

IPCP — network layer

Configure-Request / Ack

▶

IP addresses assigned — link Open

ppp + chap · cisco ios

username R2 password S3cret
!
interface Serial0/0
 encapsulation ppp
 ppp authentication chap
 ip address negotiated
 compress stac
 ppp multilink group 1

verify & troubleshoot

show interfaces serial0/0
show ppp multilink
debug ppp negotiation
debug ppp authentication